A number of thermo-expansive microcapsules, which comprise thermoplastic polymer forming their wall and low-boiling-point solvent being encapsulated in the wall as core material, have been studied. JP-B 42-26524 (the term “JP-B” as used herein means an “examined Japanese patent publication”) disclosed an overall process for producing thermo-expansive microcapsules. JP-A 62-286534 and JP-A 5-285376(the term “JP-A” as used herein means an “unexamined published Japanese patent application”) (corresponding to U.S. Pat. No. 5,536,756) disclosed the production processes for heat-resistant thermo-expansive microcapsules, wherein the heat resistance of the microcapsules is improved by forming their wall with 80% or more of acrylonitrile monomers or with monomers polymerizable into a high-glass-transition-point homopolymer to raise the initial expanding temperature of the microcapsules, and by adding a small quantity of radical polymerizable polyfunctional monomers to those wall-forming monomers to thermo-polymerize the wall-forming monomers and to harden the wall of the microcapsules when they are thermally expanded. For attaining a desirable effect in those processes, the wall-forming monomers should be crosslinked instantly with high degree of cross-linking when the microcapsules are heated.
WO99/43758 disclosed the thermo-expansive microcapsules that can be applied at high temperature. The disclosed microcapsules are imparted with heat resistance by the thermosetting resin in wall-forming material, the resin which contains functional groups reactive with carboxyl groups in the wall-forming material and hardens simultaneously with the thermo-expanding of the microcapsules. The resultant expanded microcapsules are characterized by their glassy brittle wall (shell), and thus they are completely different from elastic microcapsules. Such microcapsules are incorporated into very hard or inelastic matrices, and are not preferable to be incorporated into matrices processed into transformable porous products because the microcapsules may sometimes deteriorate the property of those resin matrices.
Thermo-expansive microcapsules similar to the above-mentioned microcapsules have been conventionally developed by employing copolymers comprising nitrile monomers and acrylate or methacrylate monomers as major components for wall-forming materials. However, the expanding temperature of the thermo-expansive microcapsules, of which wall-forming materials comprise copolymers of those monomers, is limited in a narrow range, and especially in high temperature region.